One of the hardest aspects of building a movie franchise like the Marvel Cinematic Universe is keeping the actors around. Before Marvel even got started on the third phase of its Avengers series, rumors started circulating that not every actor would be sticking around for Infinity War. Contracts expire, actors start demanding more money, and others simply just want to move on. The earliest the fourth phase could start is in 2019, with 2020 being a much more likely start date. While some of the actors will probably be sticking around, we doubt Marvel will be able to get everybody back.
Luckily, the comics have already solved this problem. Main heroes, such as Captain America, Thor, and Iron Man, have been replaced multiple times throughout their comic careers. So, movie audiences should get ready for new faces replacing familiar roles. Even if some of the characters themselves don't change, the actors probably will. It's hard to think of anyone other than Robert Downey Jr. playing Tony Stark, but people said the same thing about James Bond and Sean Connery. Just think of it like a comic book series changing its main artist. Marvel will probably try to cast younger actors so they don't have to worry about recasting for a while, so this might be a good time for someone like Asa Butterfield to start growing a goatee.